A nibble of toast, a bite of apple, and grandma says she’s full.

You may have noticed this phenomenon: As they age, between 15 and 20 percent of older adults develop appetite loss that geriatricians call “anorexia of aging.” But unlike anorexia in younger adults, this loss of appetite in older adults can be just part of the aging process, and may not be caused by psychological or genetic factors.

To better understand anorexia of aging, researchers are eying several hormones involved in controlling appetite.

For this study, researchers enrolled 31 adults ages 21 to 92. The group fasted for about 12 hours, and then consumed 660 calories of a supplemental drink. Then, the group fasted for several more hours, after which they consumed the supplemental drink until they were full.

The researchers took blood samples just before and after the participants had breakfast. The participants also rated how hungry they felt. The researchers examined the blood samples for appetite control peptide YY, an appetite stimulant hormone called ghrelin (grel-en), and a third appetite reducing hormone called peptide 1.

In the adults over 80, the appetite control peptide YY increased, but there was no increase in the other two hormones. The older adults also felt less hungry than the younger adults.

Knowing which hormone affects older adults’ lack of appetite can help researchers understand why “anorexia of aging” occurs, and perhaps how to prevent it. Malnutrition in older adults can lead to a frail immune system, muscle weakness and poor wound healing, so effective treatments could improve health.

One day, with the right hormonal adjustment, grandma may be able to finish her toast and her apple.